Devices are known for heat treating divided solids, which devices comprise at least one transfer member having at least a longitudinal axis and a helical portion mounted to rotate about said longitudinal axis inside a tubular casing, the helical portion being made of an electrically conductive material and being connected to a source of electrical power so as to constitute heating transfer means. One such device is disclosed in documents WO-A-99/39549 and FR-A-2 892 888 in the name of the Applicant.
As variants, treatment devices are also known that have a vibrating tube likewise heated by the Joule effect, as disclosed for example in documents FR-A-2 788 260 and FR-A-2 788 336.
Nevertheless, devices of those types are not suitable for treating at temperatures lying in the range 300° C. to 850° C. as is required in the field of pyrolysis where it is desired to maximize the gas phase content and thereby optimize the yield of energy densification.
In such applications, the Applicant has proposed improving the above-mentioned device by providing a tubular casing with inside walls made of refractory material, said walls then themselves constituting means for radiant heating of the mass of divided solids progressing inside the tubular casing, the intended transit times always being of the order of several tens of minutes, in particular because of the relatively slow rate of temperature rise in the material (of the order of a few tens of degrees centigrade per minute).
Under all circumstances, known pyrolysis treatments continue to have a poor operating yield, and in particular they are unsuitable for pyrolyzing a biomass in order to obtain pyrolytic oils for energy purposes.
The device of the above-mentioned documents has also been used in pyrolytic treatment specifically intended for a completely different context, namely the production of food-grade smoke, as described in document WO-A-2004/077966. Such an installation is of no advantage in the context of an application to energy densification of biomass in order to obtain pyrolytic oils for energy purposes because of the poor yield of the resulting oils, which yield may at best reach about 400 oil. Furthermore, the electrical power is used solely for heating the material up to pyrolysis temperature, such that production cost is very high.
Other pyrolysis installations are known that are designed to treat biomasses as a fluidized bed, drying them in order to increases their net calorific value, with a counterflow of a mixture of air and hot sand passing therethrough in order to achieve very fast heating of the biomass. The temperature conditions of pyrolysis are then indeed favorable for energy densification of a biomass, however the installations in question are extremely complex and it is also not possible to avoid the drawback inherent to the presence of particles of sand in the resulting pyrolytic oils, which requires filtering downstream from the installation.
In general, it appears at present difficult to implement pyrolysis of biomass in order to obtain pyrolytic oils for energy purposes while achieving simultaneously operating conditions that are favorable to such pyrolysis, good process yield, i.e. a yield of not less than about 65% oil, and reasonable production costs.